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ConversionWax Playbook

Mobile Image Format Showdown

Test mobile-optimized images vs. non-optimized images and prove what actually lifts conversions.

Effort: Medium
Time to results: 1-2 weeks
Difficulty: Easy
Best for: E-commerce, SaaS, paid campaigns

Tactic Overview

Most teams ship a single desktop-first hero image that gets resized for mobile, creating slow loads, awkward cropping, and hidden CTAs. This tactic uses device-specific A/B testing to serve mobile-optimized images that increase CTR, reduce bounce rates, and improve attribution for paid traffic.

Primary goal

Increase mobile CTR by 10-20%

Mechanism

Device-targeted image variants

Where it shows

Hero sections, landing pages

Why This Works

The friction

Desktop-first images with 16:9 ratios become heavy, slow files on mobile that cut off messaging and hide CTAs below the fold. Visitors from paid campaigns hit these sluggish pages, bounce before converting, and your attribution data can't connect creative performance to revenue.

The unlock

Mobile-optimized images use 4:5 or 3:4 aspect ratios that keep CTAs visible, leverage WebP/AVIF formats for 30-50% smaller file sizes, and pair with UTM-based targeting to measure exactly which creative drives conversions. You can test a control (desktop image resized) against a variant (mobile-first crop) and prove ROI in 1-2 weeks.

Implementation Steps

9 steps

  1. 1

    Define the test hypothesis

    Quick Win

    Set your primary KPI to mobile CTR. Hypothesize that a mobile-optimized creative (lighter file, mobile-friendly ratio, visible CTA) will increase CTR versus the current desktop image. Establish guardrails: maintain or reduce bounce rate with no drop in conversion rate.

  2. 2

    Prepare control and variant assets

    Quick Win

    Create your control (current desktop image resized, e.g. 16:9 at 1920×1080) and variant (mobile-first ratio like 4:5, 3:4, or 1:1 with CTA visible in the first 600-800px viewport). Export the variant in modern formats (WebP/AVIF) with JPEG/PNG fallbacks and ensure the crop preserves subject focus.

  3. 3

    Upload assets to ConversionWax

    Quick Win

    Upload both images to the Asset Library in ConversionWax. Name them clearly (e.g., hero_control_desktopish.jpg and hero_variant_mobile.webp) so you can easily identify each version during analysis.

  4. 4

    Target mobile devices only

    Quick Win

    Use ConversionWax's device-specific targeting to restrict the test to mobile visitors while keeping the desktop experience unchanged. Enable automatic image optimization to ensure each device receives appropriately sized files without manual intervention.

  5. 5

    Configure the A/B test split

    Medium Effort

    Set up a 50/50 split between your control (non-optimized image) and variation (mobile-optimized image). For testing three variants, use a 33/33/33 split. Plan to run the test for 1-2 weeks minimum or until you reach statistical significance at your chosen confidence level (typically 95%).

  6. 6

    Isolate traffic with UTM parameters

    Medium Effort

    Add UTM tags to your paid campaigns (e.g., ?utm_source=meta&utm_medium=cpc&utm_campaign=bfcm_hero_test&utm_content=variant). Use ConversionWax's URL variable targeting to scope the experiment to these UTM parameters, letting you compare paid cohorts versus organic traffic performance.

  7. 7

    Add optional context layers

    Medium Effort

    Layer on location-based personalization to compare performance across top cities or countries, and time-based rules to test morning versus evening visitor behavior. These additional dimensions can reveal high-performing segments worth targeting separately.

  8. 8

    Wire up multi-linking for attribution

    Medium Effort

    Use ConversionWax's multi-linking personalization to assign unique destination URLs per image and device (e.g., /offer?src=hero&v=control&m=mobile and /offer?src=hero&v=variant&m=mobile). This preserves attribution in your analytics, connecting downstream conversions directly to the exact creative and device combination.

  9. 9

    QA across devices and links

    Quick Win

    Before launching, test on multiple mobile devices and screen sizes to confirm CTAs are visible, images load quickly, and tracking links fire correctly. Verify that each variant displays as intended and that analytics events are captured for views, clicks, and conversions.

Real-World Example

Scenario

An e-commerce brand running Black Friday campaigns sent mobile traffic to a desktop hero image (16:9 ratio, 1.2MB JPEG). Mobile visitors saw a 22% bounce rate and 3.8% CTR on the hero CTA because the button was cut off below the fold and the image took 4+ seconds to load on 4G connections.

Outcome

After implementing a mobile-optimized 4:5 ratio image in WebP format (380KB), mobile CTR rose to 5.2% (+37% lift), bounce rate dropped to 16% (-6 percentage points), and conversion rate improved by +12%. Attribution via unique UTM links showed the variant drove 28% more revenue per visitor.

Key Metrics to Track

Mobile CTR

Primary success metric. Target a 10-20% lift over your control to justify mobile-first creative investment.

Bounce Rate

Should decrease by 5-10 percentage points as faster loads and visible CTAs keep visitors engaged.

Page Load Time

Modern formats (WebP/AVIF) can reduce file size by 30-50%, improving load speed on mobile networks. Monitor Largest Contentful Paint (LCP).

Conversion Rate

Guard rail metric. Ensure conversions maintain or improve as CTR increases - you want quality clicks, not just volume.

Common Pitfalls

Stopping the test too early

Random spikes can mislead you in the first few days. Run until you hit 95% confidence and sufficient sample size (typically 1-2 weeks with steady traffic) to ensure results are statistically valid.

Testing too many variables at once

If you change the image, CTA copy, and layout simultaneously, you won't know which drove the lift. Isolate image optimization first, then layer additional tests.

Ignoring attribution setup

Without unique UTM parameters and multi-linking, you can't connect creative performance to revenue. Set up tracking links before launch so you can prove ROI to stakeholders.

Forgetting mobile safe zones

Even with a mobile ratio, CTAs can be obscured by browser UI or notches. Test on real devices (iPhone, Android) to verify your CTA appears within the first 600-800px of viewport height.

Quick Reference

Best for

E-commerce, SaaS, paid campaigns with mobile traffic

Effort level

Medium

Time to results

1-2 weeks

Difficulty

Easy